This invention relates to a procedure for the refining of oils, in particular; fatty oils or triglyceride oils, in other words a procedure for the separation of impurities from these oils. Alternatively, it also relates to the recovery of these impurities or at least of constituents of these from crude oil.
In particular the invention relates to a procedure for conditioning of vegetable and animal oils, which as known mainly consist of glycerol and esters of fatty acids, for the purpose of removing a number of the impurities and to prepare these oils to be subjected to a number of further treatments, so that subsequently oil of the highest quality is obtained which is suitable for the food industry or for other industrial purposes.
Even more especially, the present invention is connected with the desliming or the "degumming" of crude oils, such as cotton seed oil, benne oil, sunflower oil, corn oil, soya bean oil, rape oil, coconut oil, palm oil, lard oil, whale oil, peanut oil, linseed oil, etc.
Indeed, it is so that crude oil or semirefined oils of the aforementioned type show a number of impurities or rather side-constituents which behave as slime substances in the oil. The separation of these side-constituents is important, not only for the commercial value of some side-constituents as by-products, but also because of the presence of certain side-constituents is generally undesired for the end use of the oil for example in the case of edible oils, a number of the side-constituents are difficult to digest and/or with the heating of the oil cause an undesired discoloration such as the formation of black flocs.
An important group of the aforementioned side-constituents are formed by the phospholipids, in other words lipids which differentiate from the neutral oil by the presence of phosphorus under the form of an esterified phosphoric acid with glycerol. The chemical reactions on which the remaining acidification can here participate, are, on the one hand, an alcoholizing esterification and, on the other hand, a salt formation with a bivalent salt such as magnesium of calcium. Under the influence of different circumstances such as harvest, origin, variety, time, temperature, humidity, etc. they can, as a result of the metabolic enzymes still acting in the crude oil, show chemical alterations in the structure of the phospholipids. Specifically a so-called salt bridge can develop between two phospholipids, mainly as follows: (only the primary chain is depicted) EQU Lipid--P--O--Mg--O--P--Lipid
or EQU Lipid--P--O--Ca--O--P--Lipid.
This reaction causes a splitting within the large group of the phospholipids. Indeed when such a salt bridge occupies the last acid function of the phosphoric acid, the phospholipid can no longer be hydrated. There is then also an important difference made between, on the one hand, hydratable (one or two acid functions possible) and, on the other hand, unhydratable phospholipids (no acid function possible).
Well then, the hydratable phospholipids may easily be removed from the oil to be refined by hydrating them, such that they become undissolvable in the oil and may be removed by means of a centrifugal gravitational separation. The hydration consists mainly of a treatment with water or steam at higher temperatures, whereby liquid crystals are formed. It is noted that the hydration reaction by itself always already occurs in a limited extent, in view of the presence of water herein. As known, a hydration reaction is actually only a weak interaction and there can hardly be a question of a real reaction. This implies that they can occur under mild conditions, but this weak interaction with an already strong bond, either covalent or ionic, cannot enter into competition.
After the hydratable phospholipids and/or other hydratable side-constituents as aforementioned are removed from the crude oil, the unhydratable side-constituents, mainly the phospholipids which have formed salt bridges, still remain present in the oil. In a large number of applications, among others with the production of edible oils, it is necessary that also these side-constituents, however limited, are removed through refining. Edible oils are generally deodorized whereby the oils are heated in the presence of steam under underpressure. Should the aforementioned side-constituents remain present in the oil, they become black during the deodorization and the aspect and the aroma are negatively influenced, such in opposition to the purpose.
In order to be able to separate the unhydratable impurities, mainly unhydratable phospholipids, from the neutral oil, two procedures are thus far known which are applicable for industrial application.
The first, most classic procedure consists in a treatment of the oil with a strong alkaline means of reaction, such as sodium hydroxide, in order to neutralize the free fatty acids present in the oil and in order to convert the unhydratable phospholipids into a hydratable form. Through this caustic treatment soapstock develops by the neutralization of the free fatty acids, which by separation on basis of force of gravity or centrifugal force or by means of classic membrane technology may be separated.
This procedure has several disadvantages. In order to improve the color of the oil, an excessive amount of alkali is generally used, with the result that a part of this results in slime substances, which accordingly agglomerate and finish up in the soapstock. The presence of this and other side-constituents in the soapstock give difficulties with the splitting up of the soap. Furthermore with the splitting up of the soap the impurities end up in the acid water, which gives rise to environmental technical problems with regard to the waste water.
At the same time the calcium and magnesium ions, which are liberated from the unhydratable phospholipids, form undissolvable phosphate bonds. The precipitated calcium and magnesium phosphates form a heavy, oleaginous precipitation, which is deposited on the drums of the centrifuges, which are used to separate the soapstock from the oil.
After the alkaline treatment and the separation of the soapstock, a number of remaining impurities are removed from the oil by adding Fuller's earth to this, after which the Fuller's earth together with the impurities is removed by filtration. The contaminated Fuller's earth forms an environmentally detrimental waste product.
After the bleaching, the aforementioned deodorization is effected, followed by possibly further other treatments.
In order to exclude the disadvantages of the conventional procedure, several variants already presented, among others as described in the U.S. Pat. No(s). 2,245,537 2,351,184, 2,576,958, 2,666,074 and 2,782,216 as well as in the French patent nos. 1,385,670 and 1,388,671. The procedures described in these patents have the disadvantage that they are either not suitable to be applied on industrial level, or that they can offer a solution to the aforementioned problems.
The second procedure for the removal of the unhydratable phospholipids, which is suitable for industrial application, is more recent and was described in the Dutch patent application no. 7709915, and is known under the name Segers process. The first step consists hereby also of a hydration followed by a centrifugal or gravitational separation. The reason for the second step is the basic observation that oils, which contain very limited or no amount of hydratable phospholipids, may be better refined when a hydratable phospholipid is added to the oil, and this phospholipid is removed from the oil together with the impurities by desliming. Preferably the oil is simultaneously treated with an acid combination of phosphoric acid and citric acid in order to promote the formation of compounds or crystals. Subsequently the formed pulp is removed by gravitational separation. At the same time other further treatments may be effected.
The great advantage of the Segers process consists in that one can reach such a low phospholipid content that the thermal capacity of the oil is so great that the treatment may be completed with the physical refining, whereby the free fatty acids, color components and aromatic components may be separated from the oil under vacuum with a thin film of steam stripping. Because of this the treatment with Fuller's earth becomes unnecessary with the result that, in view of the high cost price of the Fuller's earth, the Segers process is significantly more economical than the classic alkaline treatment. According to the Segers process oils containing phospholipids may so be treated that the remaining phosphorus content, dependent on various factors, amounts to 10 to 20 mg/kg.